Antidepressant-induced Switch of
1-Adrenoceptor Trafficking as a Mechanism for
Drug Action*
Sibylle
Bürgi,
Kurt
Baltensperger, and
Ulrich E.
Honegger
From the Department of Pharmacology, University of Bern,
CH-3010 Bern, Switzerland
Reduction in surface
1-adrenoceptor (1AR) density is thought to play
a critical role in mediating the therapeutic long term effects of
antidepressants. Since antidepressants are neither agonists nor
antagonists for G protein-coupled receptors, receptor density must be
regulated through processes independent of direct receptor activation.
Endocytosis and recycling of the 1AR fused to green fluorescent
protein at its carboxyl-terminus (1AR-GFP) were analyzed by confocal
fluorescence microscopy of live cells and complementary ligand binding
studies. In stably transfected C6 glioblastoma cells, 1AR-GFP
displayed identical ligand-binding isotherms and adenylyl cyclase
activation as native 1AR. Upon exposure to isoproterenol, a fraction
of 1AR-GFP (10-15%) internalized rapidly and colocalized with
endocytosed transferrin receptors in an early endosomal compartment in
the perinuclear region. Chronic treatment with the tricyclic
antidepressant desipramine (DMI) did not affect internalization
characteristics of 1AR-GFP when challenged with isoproterenol.
However, internalized receptors were not able to recycle back to the
cell surface in DMI-treated cells, whereas recycling of transferrin
receptors was not affected. Endocytosed receptors were absent from
structures that stained with fluorescently labeled dextran, and
inhibition of lysosomal protease activity did not restore receptor
recycling, indicating that 1AR-GFP did not immediately enter the
lysosomal compartment. The data suggest a new mode of drug action
causing a "switch" of receptor fate from a fast recycling pathway
to a slowly exchanging perinuclear compartment. Antidepressant-induced
reduction of receptor surface expression may thus be caused by
modulation of receptor trafficking routes.
*
This study was supported by Swiss National Science
Foundation Grants 32-47029.96 (to U. H.) and 3100-059124.99 (to
K. B.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
To whom correspondence should be addressed: Dept. of Pharmacology,
University of Bern, Friedbuehlstr. 49, CH-3010 Bern, Switzerland. Tel.:
41-31-632-3281; Fax: 41-31-632-4992; E-mail:
ulrich.honegger@pki.unibe.ch.
Copyright © 2003 by The American Society for Biochemistry and Molecular Biology, Inc.
